The present invention relates to heat pump systems, and is directed more particularly to an improved apparatus and method for limiting the pressure in the high pressure side of a heat pump system to a safe value, without causing the defrost control circuitry thereof to initiate unnecessary defrost cycles.
Because of concern about environmental effects, the CFC and HCFC refrigerants which were once commonly used in residential and light commercial heat pump systems are being phased out, and replaced by non-chlorinated refrigerants. One of the more promising ones of these replacement refrigerants is HFC R-410A. Unfortunately, these replacement refrigerants can establish, in the high pressure sides of the systems in which they are used, operating pressures which are up to 70% higher than those established by the refrigerants which they replace.
One way of enabling heat pump systems to operate with non-chlorinated refrigerants is to replace the piping used in the high pressure sides thereof with piping that is capable of withstanding the higher pressures that are associated with such refrigerants. In the case of a heat pump system, the cost of replacing this piping can be prohibitively expensive. This is because the high pressure side of the system includes not only the piping included in the outdoor unit, but also the piping, commonly referred to as the "line set", which connects the outdoor unit to the indoor unit, and which is usually located underground and/or within the walls of the structure to be heated.
Another way of enabling heat pump systems to operate with non-chlorinated refrigerants is to include in the heat pump system equipment which monitors the pressure within the high pressure side of the system, and takes action to limit that pressure when it equals or exceeds a threshold pressure that is less than the maximum pressure at which the system may operate safely. One example of pressure limiting equipment of this type is described in copending U.S. patent application Ser. No. 08/995,624, filed Dec. 22, 1997, which is assigned to assignee of the present invention. The latter pressure limiting equipment includes a pressure sensitive switch which monitors the pressure of the refrigerant fluid within the vapor line of the outdoor unit, and turns the outdoor fan on and off (i.e., cycles the outdoor fan) as necessary to limit the pressure in the vapor line and line set to a safe value.
While the above-mentioned pressure limiting equipment is able to solve the pressure problems associated with the use of non-chlorinated refrigerants, it can cause the heat pump system to operate inefficiently when it is operating in its heating mode at relatively high outdoor temperatures. This is because, under the latter operating condition, the temperature of the outdoor coil may fluctuate in a manner that causes the heat pump system to initiate a defrost cycle even though there is then little or no ice on the outdoor coil. Such defrost cycles not only result in the unnecessary expenditure of the energy necessary to perform them, they also prevent the heat pump system from providing the heat necessary to maintain the temperature of the space to be heated. As a result, the heat pump system must switch over to an alternative heat source which provides heat while the defrost cycle is in progress, but which does so at a cost which is significantly higher than the cost of operating the heat pump system.
In view of the forgoing, it will be seen that, prior to the present invention, the practicability of using pressure limiting equipment to enable heat pump systems to operate with non-chlorinated refrigerants, without using piping having a pressure rating higher than that used with chlorinated refrigerants, was limited by the tendency of the pressure limiting equipment to initiate unnecessary defrost cycles.